In an optical communication system, the limits of a transmission speed (bit rate of data) or a total data transmission capacity (transmission speed per channel×number of channels), and a possible transmission distance depend on an optical S/N ratio (Optical Signal-to-Noise Ratio), a Q factor, and the waveform distortion or the phase distortion of an optical signal. The waveform distortion and the phase distortion of the optical signal significantly depend on the chromatic dispersion (including higher-order dispersion) of a transmission line optical fiber, a nonlinear optical effect, etc. Moreover, the optical S/N ratio and the Q factor depend on an amplified spontaneous emission (ASE) noise caused by an optical amplifier for compensating for the loss of an optical fiber, or a noise characteristic within a transmitter or a receiver, and the like.
The following techniques for compensating for the waveform distortion of an optical signal, which is caused by chromatic dispersion, are known.
(1) A transmission line where a normal dispersion fiber and an anomalous dispersion fiber are alternately provided.
(2) A chromatic dispersion compensator such as a dispersion compensation fiber, etc.
(3) A configuration for executing electric signal processing after converting a received optical signal into an electric signal.
Up to now, a WDM (Wavelength Division Multiplexing) optical fiber transmission system for making a 10-Gbps data transmission while compensating for a transmission loss with an optical amplifier has been developed. Moreover, a higher-speed long-distance data transmission (such as 40 Gbps, 160 Gbps) and a method for providing an expandable system margin to a photonic network have been developed.
However, the optical S/N ratio and the Q factor are seriously degraded by an ASE noise caused by an optical amplifier, or the like even if dispersion compensation and distortion compensation of high precision, and an optical amplifier with high quality are combined. Therefore, a practical transmission distance is limited. To realize a long-distance optical fiber transmission of a high-speed signal, the demand for a technique of shaping a distorted optical waveform, a technique of correcting a phase distortion, and a technique of suppressing accumulated ASE noise, phase noise, etc. has been rising.
Additionally, in an optical network that functions as a communication backbone for future ultra-large capacity information, it is desired to realize an optical node device that can flexibly process the above described high-speed optical signal and is implemented by combining an optical switch, a wavelength converter, etc. Accordingly, the development of an optical signal processing device less degrading the optical S/N ratio, and a technique of improving the optical S/N ratio has been demanded.
As a technique of shaping the waveform of an optical signal, an optical waveform shaping device having first and second power controllers and a nonlinear optical medium is known. The first power controller controls the power of signal light. The second power controller controls the power of pumping light having a wavelength different from the signal light. To the nonlinear optical medium, the signal light the power of which is controlled by the first power controller, and the pumping light the power of which is controlled by the second power controller are input. The first power controller controls the power of the signal light so that a gain produced by the pumping light is saturated in the nonlinear optical medium. As a result, an optical limiter function is realized, and an optical waveform is shaped.
As related art, Japanese Laid-open Patent Publication No. 2007-264319, Japanese Laid-open Patent Publication No. 2006-184851, and Japanese Laid-open Patent Publication No. 2000-75330 are proposed.
In the conventional technology, the optical S/N ratio and the Q factor cannot be improved without changing a waveform and a spectrum. Moreover, with a method for shaping the waveform of an optical signal with an optical limiter, the amplitude noise of an ON level of the optical signal can be suppressed, but the noise of a zero level cannot be suppressed.